Epoxy optical sheet and process for producing the same

ABSTRACT

An epoxy optical sheet having a thickness of 500 μm or less, a high thickness accuracy, a small retardation and an excellent heat resistance; and a process by which the optical sheet can be efficiently produced.  
     An epoxy optical sheet ( 8 ) comprises a hardened sheet ( 62 ) comprising an epoxy resin, and another resin layer ( 31 ) formed on one side thereof, wherein the sheet has a retardation of 5 nm or less, an average thickness of 500 μm or less, a thickness accuracy of ±10% or less and a glass transition temperature of  170 ° C. or above, and the sheet has a smooth surface; and a process for continuously producing the epoxy optical sheet ( 8 ), which comprises successively forming a strippable resin layer ( 31 ) on a support ( 1 ) having a smooth surface, successively spreading an epoxy resin coating solution ( 6 ) comprising an epoxy resin, a hardener, a hardening accelerator and a leveling agent on the resin layer in the form of a sheet, followed by hardening treatment, thereby forming a hardened sheet adhering to the resin layer; and, at the same time, recovering the hardened sheet together with the resin layer from the support.

FIELD OF THE INVENTION

[0001] This invention relates to an epoxy optical sheet excellent inoptical properties and heat resistance, and a process for producing theepoxy optical sheets at high mass-productivity.

BACKGROUND OF THE INVENTION

[0002] Conventional optical sheets made of epoxy resins are thoseobtained by pouring an epoxy resin into a space of a cast mold followedby hardening (JP-A-6-337408, JP-A-7-28043 and JP-A-7-120741; the term“JP-A” as used herein means an “unexamined published Japanese patentapplication”) However, it is difficult by this casting method to obtaina sheet having a thickness of 500 μm or less and being free from anyoptical strain.

[0003] In a method wherein a hardened plate made of an epoxy resin iscut into a sheet, optical strain generates in cutting step when athickness is 500 μn or less. As a result, it was also difficult toobtain an optical strain-free sheet by this method. Thus, epoxy opticalsheets having high thickness accuracy at a thickness of 500 μm or less,small retardation and excellent heat resistance have not yet beenprovided, in spite of a demand therefor.

[0004] Moreover, the above-described method suffers from a disadvantagethat the complicated steps such as casting and mold opening arerequired, and this results in poor production efficiency of epoxyoptical sheets. Thus, a method is considered, which continuouslyproduces epoxy optical sheets by, for example, a calender method usingrolling rolls, a melt extrusion method with T-die or the like, or a flowcasting method of a resin solution onto a support, according to theconventional resin sheet production.

[0005] However, the epoxy resin involves use as adhesives. Therefore,when the epoxy resin is molded into a sheet by these methods, the epoxyresin adheres to rolls, dies, supports, etc. As a result, it isimpossible to easily separate and recover the molded sheets, resultingin breakage of the sheet in separation thereof. In addition, therearises another problem that the thus obtained sheets tend to have unevensurface and thus fail to achieve the required surface smoothnessrequired in optical sheets.

SUMMARY OF THE INVENTION

[0006] Accordingly, one object of the present invention is to provide anepoxy optical sheet having a thickness of 500 μm or less, an excellentthickness accuracy, a small retardation and a high heat resistance.

[0007] Another object of the present invention is to provide a processfor producing the epoxy optical sheet at high efficiency.

[0008] The present invention provides an epoxy optical sheet comprisinga hardened sheet comprising an epoxy resin, and another resin layerformed on one side thereof, wherein the sheet has a retardation of 5 nmor less, an average thickness of 500 μm or less, a thickness accuracy of±10% or less, a glass transition temperature of 170° C. or above and asmooth surface.

[0009] The present invention further provides a process for continuouslyproducing an epoxy optical sheet, which comprises successively forming astrippable resin layer on a support having a smooth surface,successively spreading an epoxy resin coating solution comprising anepoxy resin, a hardener, a hardening accelerator and a leveling agentonto the resin layer in the form of a sheet, followed by hardeningtreatment, thereby forming a hardened sheet adhering to the resin layer;and at the same time, recovering the hardened sheet together with theresin layer from the support.

BRIEF DESCRIPTION OF THE DRAWING

[0010] The Figure is a view showing the production process according tothe present invention, wherein 1 is an endless belt (support), 31 is aresin layer, 61 is a spread layer of an epoxy resin coating solution, 7is a hardening device, 62 is a hardened sheet and 8 is an epoxy opticalsheet.

DETAILED DESCRIPTION OF THE INVENTION

[0011] According to the production process of the present invention, theepoxy optical sheet having the strippable resin layer can be separatedfrom the support and thus recovered. Therefore, the surface conditionsof the support can be appropriately transcribed and reflected via theresin layer. As a result, sheets having excellent optical properties,such as mirror surface, can be continuously and efficiently produced bya series of convenient procedures. Also, the mass production speed canbe easily controlled by regulating the moving rate of the spread layermediated by the support. Moreover, the thickness of the sheet can beeasily controlled by regulating the moving rate and the spread of thecoating solution. Consequently, it is possible to obtain an epoxyoptical sheet having an average thickness of 500 μm or less, a highthickness accuracy, a small retardation and an excellent heatresistance.

[0012] The epoxy optical sheet of the present invention comprises ahardened sheet comprising an epoxy resin, and another resin layer formedon one side thereof, wherein the sheet has a retardation of 5 nm orless, an average thickness of 500 μm or less, a thickness accuracy of±10% or less, a glass transition temperature of 170° C. or above, and asmooth surface. FIG. 1 shows an example of the production process of thesheet, wherein 8 is an epoxy optical sheet, 31 is a resin layer, and 62is a hardened sheet of an epoxy resin.

[0013] The process for producing an epoxy optical sheet according to thepresent invention comprises, for example, successively forming astrippable resin layer on a support having a smooth surface;successively spreading an epoxy resin coating solution comprising anepoxy resin, a hardener, a hardening accelerator and a leveling agentonto the resin layer in the form of sheet, followed by hardeningtreatment, thereby forming a hardened sheet adhering to the resin layer,and at the same time, recovering the hardened sheet together with theresin layer from the support. According to this process, an epoxyoptical sheet can be continuously produced at high mass productivity.

[0014] The Figure shows an example of the production step by the aboveprocess. In this process, continuous production is performed by the flowcasting method. A support comprising an endless belt 1 having a smoothsurface is driven at a definite speed (for example, 0.1 to 10 m/minute,preferably 0.2 to 5 m/min) via a driving drum 11 and a coupled drivingdrum 12. During this operation, a resin solution 3 is continuouslyapplied on the support via a die 2, dried and hardened optionally underheating or by irradiation with light to obtain a film 31. In thisFigure, an ultraviolet radiator 4 is disposed.

[0015] While successively forming the resin layer 31 on the support asdescribed above, an epoxy resin coating solution 6 is successivelyapplied on the resin layer via a die 5 and spread into the form of asheet. The spread layer 61 is then hardened with the use of anappropriate hardening device 7 of the heating or irradiation type. Thusa hardened sheet 62 adhering to the resin layer 31 is successivelyformed and, at the same time, separated together with the resin layer 31from the support 1, thus continuously producing an epoxy optical sheet8.

[0016] From the viewpoint of preventing cracking, etc., it is preferableto recover the epoxy optical sheet from the support in a hightemperature atmosphere exceeding the glass transition temperature.Therefore, it is preferable to perform the recovery after hardening theepoxy resin through epoxy group has proceeded at an extent of about 70%or more and the resin has been hardened so as not to undergo plasticdeformation even in a high temperature atmosphere (i.e., at thehardening temperature in the case of heat hardening or at around theglass transition temperature), thus preventing cracking or deformationas described above.

[0017] In recovering the epoxy optical sheet from the support, anappropriate stripping means may be used if necessary. The thus formedcontinuous epoxy optical sheet can be cut into pieces of appropriatesize, if necessary, using laser beams, an ultrasonic cutter, a dicer orwater jet followed by recovery.

[0018] In the above-described process, an appropriate support, forexample, a belt such as an endless belt, a plate or a drum, can be usedso long as the epoxy resin coating solution can be successively andcontinuously spread thereon and the spread layer is supported therebyand thus maintained in the form of a sheet. The support may be made ofany material so long as it can withstand the treatment for hardening theepoxy resin. Therefore, preferred examples are, for example, metals suchas stainless steel, copper and aluminum, glass and plastics. Of those,stainless steel is preferable from the standpoint of durability or thelike.

[0019] It is preferable that the surface of the support is as smooth aspossible from the point of improvement in thickness accuracy or thelike. For example, an epoxy optical sheet having a mirror surface can beobtained by using a support with a surface roughness (Ra) of 0.02 μm orless. Therefore, in the course of the formation of the resin layer orthe hardened sheet, it is preferable to maintain the surface of thesupport as horizontally as possible. Thus, an epoxy optical sheet havinga thickness accuracy of ±10% or less can be easily obtained.

[0020] To successively form the resin layer on the support, anyappropriate strippable transparent resin can be used without limitationso long as the resin never or little adheres to the support and can beeasily stripped therefrom. Examples of such a resin include urethaneresins, acrylic resins, polyester resins, polyvinyl alcohol resins (forexample, polyvinyl alcohol/ethylene vinyl alcohol copolymer), vinylchloride resins and vinylidene chloride resins.

[0021] To form the resin layer, it is also possible to use polyarylateresins, sulfone resins, amide resins, imide resins, polyether sulfoneresins, polyether imide resins, polycarbonate resins, silicone resins,fluororesins, polyolefin resins, styrene resins, vinylpyrrolidoneresins, cellulose resins, acrylonintrile resins, etc. It is alsopossible to use a blend of two or more appropriate transparent resins inthe formation of the resin layer.

[0022] The resin layer, which is stripped together with the sheet formedabove and thus functions as the surface layer in one side of the epoxyoptical sheet, is preferably one having excellent optical propertiessuch as transparency. Taking these optical properties, thestrippability, in particular, to a stainless support, etc., intoconsideration, it is preferable to use an urethane resin for theformation of the resin layer.

[0023] As described above, the resin layer functions as the surface coatlayer of the epoxy optical sheet. From this point of view, the materialof the resin layer may be selected so as to impart various functions tothe sheet, for example, chemical resistance, surface hardness, opticalanisotropy, low water absorption, low moisture-permeability and gasbarrier properties such as low oxygen-permeability. Accordingly, theresin layer may have either a single-layered structure or a laminatedstructure wherein, for example, a polyvinyl alcohol resin layer aimingat imparting gas barrier properties is formed on an urethane resin layeraiming at imparting strippability.

[0024] The resin layer may be formed by, for example, optionallydissolving the resin in an appropriate solvent (organic solvent, water,etc.), applying the resulting solution onto the definite side of thesupport in an appropriate manner (roll coating, wire bar coating,extrusion coating, curtain coating, spray coating, etc.), drying it ifneeded, and then hardening the same by a method suitable depending onthe resin (heating, light irradiation, etc.).

[0025] Where the above coating method is used, the viscosity of theresin solution is preferably regulated to 1 to 100 centipoise so as toachieve uniform coating. When the resin solution is applied to theendless belt by the flow casting method as described above, theextrusion coating method is preferable from the viewpoint of the coatingefficiency, etc. In this case, it is preferable to regulate theviscosity of the resin solution to 1 to 10 centipoise. When a coatinglayer of an urethane resin, etc., is to be hardened by lightirradiation, it is preferably to use a high-pressure or low-pressure UVlamp having a central wavelength of 365 nm or 254 nm so as to achieve anefficient treatment.

[0026] The thickness of the resin layer thus formed can be appropriatelydetermined. To achieve high strippability and prevent cracking in thestep of stripping, the thickness of each unit layer is generally from 1to 10 μm, preferably 8 m or less, and more preferably from 2 to 5 μm.

[0027] To prepare the epoxy resin coating solution to be spread onto theresin layer, at least an epoxy resin, a hardener, a hardeningaccelerator and a leveling agent are used. The epoxy resin is notparticularly limited and appropriately selected depending on, forexample, the purpose of use of the epoxy optical sheet to be produced.

[0028] Examples of the epoxy resin include bisphenol resins (bisphenolA, bisphenol F, bisphenol S, etc., each optionally hydrogenated),novolak resins (phenol novolak, cresol novolak, etc.),nitrogen-containing cyclic resins (triglycidyl isocyanurate, hydantoin,etc.), alicyclic resins, aliphatic resins, aromatic resins (naphthalene,etc.), glycidyl ether resins, low-water absorptive resins (biphenyl,etc.), dicylco resins, ester resins, ether ester resins andmodifications thereof.

[0029] From the viewpoint of optical properties such as transparency, itis preferable to use as the epoxy resin, for example, an alicyclic resinwhich contains no conjugated double bond, as in benzene ring, andscarcely undergoes color change. In general, an epoxy resin having anepoxy equivalent of from 100 to 1,000 and a softening point of 120° C.or below is preferably used from the standpoint of the mechanicalproperties (flexibility, strength, etc.) of the obtained optical sheet.Moreover, it is preferable to use a two-part liquid system which is inthe form of a liquid at the coating temperature or below, in particular,at ordinary temperature so as to obtain en epoxy resin coating solutionbeing excellent in the coatability and spreading property into a sheet.Epoxy resin may be used alone or as a mixture of two or more thereof.

[0030] The hardener is not also particularly limited. Either onehardener or a mixture of two or more thereof may be used depending onthe epoxy resin. Examples of the hardener include organic acid compounds(tetrahydrophthalic acid, methyltetrahydrophthalic acid,hexahydrophthalic acid, methylhexahydrophthalic acid, etc.) and aminecompounds (ethylenediamine, propyelnediamine, diethylenetriamine,triethylenetetramine, amine adducts thereof, methaphenylenediamine,diaminodiphenylmethane, diaminodiphenylsulfone, etc.).

[0031] Further examples of the hardener include amide compounds(dicyandiamine, polyamide, etc.), hydrazide compounds (dihydrazide,etc.) and imidazole compounds (methylimidazole,2-ethyl-4-methylimidazole, ethylimidazole, isopropylimidazole,2,4-dimethylimidazole, phenylimidazole, undecylimidazole,heptadecylimidazole, 2-phenyl-4-methylimidazole, etc.).

[0032] Still further examples of the hardener include imidazolinecompounds (methylimidazoline, 2-ethyl-4methylimidazoline,ethylimidazoline, isopropylimidazoline, 2,4-dimethyl-imidazoline,phenylimidazoline, undecylimidazoline, heptadecylimidazoline,2-phenyl-4methylimidazoline, etc.), phenol compounds, urea compounds andpolysulfide compounds.

[0033] In addition, acid anhydrides, etc., can be used as the hardener.Those acid anhydrides are preferably used from the standpoints ofworking environment due to low irritativeness, durability to hightemperatures of the obtained sheet due to improvement in heatresistance, prevention of discoloration, etc. Examples of those acidanhydrides include phthalic anhydride, maleic anhydride, trimelliticanhydride, pyromellitic anhydride, nadic anhydride, glutaric anhydride,tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride,hexahydrophthalic anhydride, methylhexahydrophthalic anhydride,methylnadic anhydride, dodecenylsuccinic anhydride, dichlorosuccinicanhydride, benzophenonetetracarboxylic anhydride and chlorendicanhdride.

[0034] Of those, it is preferable to use colorless or pale yellow acidanhydride hardeners having a molecular weight of from about 140 to about200, such as phthalic anhydride, tetrahydrophthalic anhydride,hexahydrophthalic anhydride or methylhexahydrophthalic anhydride.

[0035] The amount of the hardener to be added is appropriatelydetermined depending on the type thereof, the epoxy equivalent of theepoxy resin, etc. That is, the amount is determined according to thecase of the conventional hardening of epoxy resins. In the case of theacid anhydride hardeners, for example, it is used in the proportion of0.5 to 1.5 equivalents, preferably 0.6 to 1.4 equivalents and morepreferably 0.7 to 1.2 equivalents, per equivalent of the epoxy group,from the standpoint of the color tint of the sheet obtained andpreventing decrease in moisture resistance. Even if the hardener is usedalone or two or more thereof, the amount is according to the aboveequivalent ratio.

[0036] Similarly, the hardening accelerator is not particularly limited.There are used one or more appropriate ones selected from, for example,tertiary amines, imidazoles, quaternary ammonium salts, organic metalsalts, phosphorus compounds and urea compounds, depending on the epoxyresin and the hardener used. Use of the hardening accelerator makes itpossible to accelerate the hardening speed, thereby shortening thehardening time. As a result, the length of the support can be reduced toone by several times compared with the case that the hardeningaccelerator is not used. Therefore, the amount of the hardeningaccelerator to be added is determined depending on the acceleratingeffect, etc. In general, the hardening accelerator is added in an amountof from 0.05 to 7 parts by weight, preferably form 0.1 to 5 parts byweight, and more preferably form 0.2 to 3 parts by weight, per 100 partsby weight of the epoxy resin from the standpoint of prevention ofdiscoloration or the like.

[0037] The leveling agent is added to form a smooth surface bypreventing the surface from matting by uneven surface tension due toscattering of the hardener, etc. in the step of hardening the spreadlayer of the epoxy resin coating solution while being in contact withair. For example, there are used one or more appropriate substancescapable of lowering surface tension, such as various surfactants (forexample, silicone, acrylic and fluorinated surfactants).

[0038] In preparing the epoxy resin coating solution, it is alsopossible to add, if necessary, appropriate components which aresometimes used in epoxy resin hardened products, for example,antioxidants (phenols, amines, organosulfur compounds, phosphines,etc.), modifiers (glycols, silicones, alcohols, etc.), antifoamingagents, hydroxyl-containing compounds, dyes, discoloration inhibitors,and UV absorbers. The antifoaming agents are added in order to preventthe inclusion of bubbles which deteriorate the optical properties of thesheet obtained. Polyhydric alcohols such as glycerol can be preferablyused therefor.

[0039] The epoxy resin coating solution can be prepared by making thecomponents flowable or spreadable optionally with the use of solvents.Therefore, the epoxy resin coating solution can be spread by a methodappropriate for the formation of the resin layer as described above (forexample, curtain coating, roll coating) by which the epoxy resin coatingsolution can be spread and molded into a sheet. In the case of flowcasting, it is preferred to employ the extrusion coating method from theviewpoint of the coating efficiency, etc.

[0040] The epoxy optical sheet according to the present inventioncomprises a hardened sheet comprising an epoxy resin and shows a highheat resistance such that a glass transition temperature is 170° C. orabove. Thus, it is usable as a liquid crystal cell substrate, etc.,withstanding high temperature atmospheres encountering in the process ofproducing liquid crystal cells. This glass transition temperature is avalue measured by the thermomechanical analysis (TMA) of the tensilemode under elevating temperature at a rate of 2° C. /minute.

[0041] The epoxy optical sheet has a thickness of 500 μm or less,preferably from 100 to of 400 μm and more preferably from 200 to 300 μm,from the viewpoints of rigidity (such as flexural strength), surfacesmoothness, low retardation, thin and lightweight characteristics, etc.Also the epoxy optical sheet has a thickness accuracy of ±10% or less,by taking the optical purposes of use, etc., into consideration. Thethickness accuracy is determined based on the difference between themaximum value and the average thickness and between the minimum valueand the average thickness by measuring the thickness of the sheet at 10points at regular intervals in the width direction and at intervals of60 mm in the length direction.

[0042] The epoxy optical sheet shows a retardation of 5 nm or less fromthe viewpoint of preventing coloration due to birefringence when it isapplied to a liquid crystal cell, etc. The retardation can be examinedwith a birefringence measurement device.

[0043] The epoxy optical sheet according to the present invention canpreferably be used in various optical purposes, for example, liquidcrystal cell substrates and anti-reflection sheets. Because of beingexcellent in optical properties, heat resistance and surface smoothness,the epoxy optical sheet can particularly preferably used in opticalpurposes where a high heat durability, a high flexural strength, a smallretardation and a light weight are required, for example, liquid crystalcells.

[0044] The present invention is described in more detail by reference tothe following examples, but it should be understood that the inventionis not construed as being limited thereto. Unless otherwise indicated,all parts, percents, ratios and the like are by weight.

EXAMPLE 1

[0045] An epoxy resin coating solution was prepared by mixing understirring 100 parts of 3,4-epoxycyclohexylmethyl-3,4epoxycyclohexanecarboxylate, 125 parts of methylhexahydrophthalic anhydride, 3.75 partsof tetra-n-butylphosphonium o,o-diethylphosphorodithioate, 2.25 parts ofglycerol and 0.07 parts of a silicone surfactant (leveling agent,Disparon LS-009, manufactured by Kusumoto Kasei K.K.) and aging theresulting mixture at 49° C. for 90 minutes.

[0046] A 17 wt % solution of an urethane UV-hardenable resin (NK OligoUN-01, manufactured by Shin-Nakamura Kagaku K.K.) in toluene wasdischarged from a die by the flow casting method as shown in FIG. 1 andapplied onto an endless belt rotationally driven at a definite speed(0.2 m/minute). After evaporating the toluene, the resin was hardened byUV-irradiation (central wavelength: 254 nm, accumulative quantity oflight: 2,000 mJ/cm²) to thereby form an urethane resin layer having awidth of 500 mm and a thickness of 2 μm.

[0047] Subsequently, the above epoxy resin coating solution wascontinuously discharged at a rate of 100 g/minute onto the hardenedurethane resin layer and spread into a sheet, while continuing the aboveprocedure. The spread layer was hardened by heating through a heater at120° C. for 30 minutes. The hardened sheet was separated and recovered,together with the urethane resin layer adhering thereto, from theendless belt on a coupled driving drum adjusted to 150° C. Thethus-recovered sheet was then cut into pieces at intervals of 490 mm inthe flow direction, thereby continuously obtaining epoxy optical sheetshaving a width of 490 mm and an average thickness of 400 μm, which had athickness accuracy of ±40 μm or less, a retardation of 5 nm or less anda glass transition temperature of 170° C. or above.

EXAMPLE 2

[0048] Example 1 was followed except for using a fluorine surfactant(Defenser MCF-323, manufactured by Dainippon Ink and Chemicals Corp.) inplace of the silicone surfactant, thereby continuously obtaining epoxyoptical sheets of 400 μm in average thickness which had a thicknessaccuracy of ±40 μm or less, a retardation of 5 nm or less and a glasstransition temperature of 170° C. or above.

EXAMPLE 3

[0049] Example 1 was followed except for using an acrylic surfactant(Disparon L-1980, manufactured by Kusumoto Kasei K.K.) in place of thesilicone surfactant, thereby continuously giving epoxy optical sheets of400 μm in average thickness, which had a thickness accuracy of ±40 μm orless, a retardation of 5 nm or less and a glass transition temperatureof 170° C. or above.

EXAMPLE 4

[0050] Example 1 was followed except for discharging the epoxy resincoating solution at a rate of 75 g/minute, thereby continuouslyobtaining epoxy optical sheets of 300 μm in average thickness, which hada thickness accuracy of ±30 μm or less, a retardation of 5 nm or lessand a glass transition temperature of 170° C. or above.

Comparative Example 1

[0051] Example 1 was followed except for adding no silicone surfactant.The sheet obtained showed a matt surface, which made it unusable as anoptical sheet.

Comparative Example 2

[0052] Example 1 was followed except for hardening the spread layer ofthe epoxy resin coating solution at 120° C. for 15 minutes. The epoxyoptical sheet obtained had a glass transition temperature lower than170° C. When the sheet was used as a cell substrate in the constructionof a liquid cell display device, it became warped due to theinsufficient heat resistance thereof.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof What is Claimed Is:
 1. Anepoxy optical sheet comprising a hardened sheet comprising an epoxyresin, and another resin layer formed on one side thereof, wherein thesheet has a retardation of 5 nm or less, an average thickness of 500 μmor less, a thickness accuracy of ±10% or less and a glass transitiontemperature of 170° C. or above, and the sheet has a smooth surface. 2.The epoxy optical sheet as claimed in claim 1, wherein said epoxy resinis an alicyclic type.
 3. A process for continuously producing an epoxyoptical sheet, which comprises successively forming a strippable resinlayer on a support having a smooth surface; successively spreading anepoxy resin coating solution comprising an epoxy resin, a hardener, ahardening accelerator and a leveling agent on said resin layer in theform of a sheet, followed by hardening treatment, thereby forming ahardened sheet adhering to said resin layer; and, at the same time,recovering said hardened sheet together with said resin layer from thesupport.
 4. The production process as claimed in claim 3, wherein saidhardener is acid anhydride compounds.
 5. The production process asclaimed in claim 3, wherein said epoxy resin coating solution containsan antifoaming agent.